The invention relates to a riser tensioning system for a floating oil or gas production platform.
In particular, the invention relates to a riser tensioning system for use on a deep draft floating production facility of the type illustrated in PCT Application WO99/10230. However, the invention could also be used on other floating platforms.
Oil and gas production is taking place in progressively deeper water. In water depths up to about 300 m in the North Sea and about 400 m in the Gulf of Mexico, fixed platforms have been used. In deeper water depths, floating platforms are necessary. Production has taken place from ship shaped vessels, column stabilised semi-submersible vessels, floating spars and tension leg platforms (TLPs).
In all cases, near vertical pipelines bring the oil (or gas) up from the sea bed to the floating platform for processing and onward transmission. These near vertical pipelines are known in the offshore industry as xe2x80x98risersxe2x80x99. A problem exists in that risers need to be held constantly in tension against vertical motions (xe2x80x98heavexe2x80x99) of a floating platform. If the risers are allowed to go into compression, buckling may occur. Thus it has been necessary to use heave compensators to keep the risers under tension.
In water depths greater than 1500 m, the heave period becomes a problem for TLPs. Deep Draft Floaters (DDFs) have smaller motions than conventional semi-submersible vessels, but larger motions than TLPs.
In some floating platforms, such as in xe2x80x98sparxe2x80x99 platforms, it has been known to use external buoyancy cans to tension the risers. This technique is described in U.S. Pat. No. 4,702,321. Tensioning with external cans has several drawbacks. The risers are confined in a central vertical duct. Damage from fatigue may be experienced by the risers due to uncontrolled xe2x80x98pistonxe2x80x99 actions from buoyancy cans and excitation of various modes of vibrations, as well as uncontrolled sticktion phenomena. This may lead to rupture and consequential leakage, fire and explosion with resulting damage to the topside facilities and to other risers. This makes caisson type vessels especially vulnerable. In these vessels, the leakages pass up through the caisson well into the middle of the topside deck installation. TLPs do not have this disadvantage as their risers are suspended freely in the water. In most cases a leakage in the riser system will be dispersed from the TLP by water currents and winds at the surface.
In principle, it is possible to extend (lengthen) tensioner systems developed for TLPs to accommodate the larger heave motions which are likely to be experienced by risers on DDFs and other vessels. However, this creates practical difficulties.
DDFs have slightly less air gap than TLPs between their lowest deck and the sea surface, because there is no xe2x80x9cpull-downxe2x80x9d from the tethers as the TLP moves off its nominal position. The same effect increases the need for riser xe2x80x9cpay-outxe2x80x9d for a DDF for the same displacement Additionally, for DDFs, there is a contribution from their significantly larger heave motions. To allow for this larger pay-out/pay-in of risers, (often referred to as heave compensation), the traditional xe2x80x98pulling cylinderxe2x80x99 design of heave compensator would become so long that under normal operation, the xe2x80x98tensioner ringxe2x80x99 would be partly below water level. The tensioner ring is an assembly connecting the tensioner rods of the heave compensator to the riser. If the tensioner ring is partly below water, this critical connection is difficult to reach for inspection.
To raise this critical connection to above sea level, it would be necessary for the tensioner rods to be longer, so that they would extend up through the deck opening. This would lead to a complex arrangement, with a risk of potential clashes, or loss of valuable area on the production deck or drilling equipment deck. Another expedient for raising the tensioning ring would be to invert the tensioner system, so that it had a xe2x80x98rods upxe2x80x99 configuration. This would increase the Xmas tree height above the tree deck; lead to instability in shear and torsion; and possibly lead to a compression/buckling problem with the inverted tensioner rods.
In any case, the tensioner stroke necessary for such longer tensioner systems could be beyond what is practical, reliable and cost effective.
Thus there is a requirement for a riser tensioning system which would be applicable to vessels with larger heave motions than TLPs, and which would avoid the practical difficulties outlined above.
The theoretical background to this invention is described in OTC Paper 11904 (published at Houston Tex. in May 2000).
The invention provides, in a substructure for a floating oil or gas production platform, an arrangement to tension a plurality of risers extending from the sea bed up to the substructure, the arrangement comprising:
i) a conventional hydraulic tensioner/heave compensator for each riser, in which there is a soft spring formed by a piston cylinder combination acting against an accumulator, the heave compensators for the risers being disposed to compensate for vertical oscillations of relatively short period (e.g. from 1 second to about 5 minutes) between the risers and a vertically adjustable Xmas tree deck, and
ii) a vertical position adjustment system capable of intermittent operation to adjust the vertical position of the Xmas tree deck relative to the floating substructure to compensate for longer term changes which would otherwise cause the individual riser""s tension or stroke position to depart from its target value/range; the Xmas tree deck vertical position adjustment system being normally located in one particular position within its range of movement to compensate for the longer term changes.
In the foregoing, examples of the relatively short period oscillations referred to in i) are the first order wave motions and normal operational state surge, sway and pitch slow drift oscillations. Examples of the longer-term changes referred to in ii) are an extreme quasistatic horizontal offset caused by severe storm conditions, extreme overlaid oscillations at the critical surge/sway period of the moored substructure (slow drift), or inadvertent flooding of one of the buoyant compartments of the substructure.
It is preferred that the vertical position adjustment system includes stiff hydraulics (in which pistons may be hydraulically locked) which Interconnect the Xmas tree deck and the substructure.
It is further preferred that hydraulic oil is supplied from pressurized accumulators when raising the Xmas tree deck, and bled to a tank when lowering the Xmas tree deck.
In one preferred form the Xmas tree deck has counterbalance means, such that its vertical movements to compensate for longer term changes are counterbalanced, and only minimal force is required to effect vertical movement.
In this form it is preferred that the Xmas tree deck vertical position adjustment system comprises at least three piston cylinder and accumulator combinations acting between the Xmas tree deck and the floating substructure, and in which the three combinations are synchronised to avoid excessive tilt of the Xmas tree deck relative to the substructure.
It is further preferred that the cylinders in the combinations are connected to a single accumulator, so that the Xmas tree deck is sensibly horizontal, and in which there is a rack and pinion mechanism which engages with the substructure to maintain parallellity of the moving X-mas tree deck with the substructure at all times, where rack and pinions engage at least two faces of the deck, at right angles.
In this form it is alternatively preferred that the vertical position adjustment system comprises at least three pulley systems acting between the Xmas tree deck and the substructure, and in which the pulley systems are powered to compensate for longer term vertical changes.
It is further preferred that a part of each pulley system is engaged by a further piston cylinder combination.
The pulley systems may be powered by hydraulic or electric motors for synchronous movement.
In forms of the invention wherein the Xmas tree deck has counterbalance means, it is preferred that there is means whereby synchronism can be effected by hydraulic valve logic while the vertical position adjustment system is moving.
It is further preferred that a locking provision on the vertical position adjustment system is arranged to become unlocked when a heave compensator is approaching the end of its stroke.
It is still further preferred that predetermined high and low pressures in the heave compensators are arranged to open valves between the piston cylinder combinations and the accumulators in the vertical position adjustment system.
In another form of the invention, it is preferred that the vertical position adjustment system includes mechanical engagement devices which interconnect the Xmas tree deck and the substructure.
In any of the forms of the invention described above, there may be a control system which has provision for the arrangement to operate without human intervention (e.g. in circumstances in which the floating oil or gas platform is temporarily de-manned during a hurricane).
It is preferred that the control system includes a program to adjust the elevation of the Xmas tree deck in response to stroke measuring devices on at least three of the individual heave compensators, whereby, at preset limits of compensator stroke, the vertical position adjustment system moves the Xmas tree deck in a sense towards the limit reached on the individual heave compensator.
It is further preferred that the spring rates of the individual heave compensator are increased near both the limits of travel of the individual heave compensators, such that the equilibrium of the balanced Xmas tree deck will be changed so that the Xmas tree deck moves towards the applicable limit of travel under the action of the vertical position adjustment system.
In any of the forms of the invention described above, the balanced vertical position adjustment system under mean force equilibrium may be normally retained by frictional forces in one particular position within its range of movement, and is moved intermittently in direct response to one or several of the heave compensators approaching a limit of operation.
It is preferred that hydraulic cylinders in the vertical position adjustment system are pre-pressurised, so that the system acts as a precompressed spring which fails to xe2x80x98safexe2x80x99 if the active drive systems lose pressure.
It is further preferred that the heave compensators have an increased vertical spring stiffness as they approach the ends of their stroke ranges.
Preferably, there is adjustment means to change the characteristics of individual heave compensators, so that both the heave compensators for the risers and the vertical position adjustment system for the Xmas tree deck approach their limits of operation at the same time.
In any of the forms of the invention described above the Xmas tree deck may have an integral deck centralisation system.
It is preferred that the Xmas tree deck is supported on at least four pairs of vertical position adjustment systems disposed generally symmetrically about the deck, whereby to centralise the deck within a generally horizontal aperture in the substructure, such that individual heave compensators react lateral loads from individual risers into the Xmas tree deck, and the Xmas tree deck as a whole is centralised within the horizontal aperture.
It is further preferred that vertical rods guide the Xmas tree deck within the horizontal aperture, or that projections from the Xmas tree deck engage vertical guide rails surrounding the horizontal aperture, or that there are pinions on the Xmas tree deck arranged to engage vertical racks round the horizontal aperture.
In a form in which there are pinions, it is further preferred that resilient means are disposed to hold the pinions in engagement with the racks.
In any one of the forms of the invention described above the vertical position adjustment system for the Xmas tree deck may have a generally central slot occupied by dillstring riser tensioner, so to facilitate drilling/workover.
The Xmas tree deck may be used as a foundation for a drilling riser tensioner, or for a workover drillstring tensioner.
Conveniently, hoses from individual Xmas trees on the Xmas tree deck are led through individual downwardly opening trumpet sleeves dependant from a platform above the Xmas tree deck.
Optionally, there are several individual bays of deck grid systems placed onboard the substructure to reduce the pitch differential across the riser array.
Advantageously, there is provision to lock off the vertical position adjustment system for the Xmas tree deck, whereby to adjust the vertical heave stiffness of the substructure.
The invention includes a substructure for an oil or gas production platform, and having an arrangement as described above.
The invention also includes a method of controlling the tension in risers extending from the sea bed up to the hull of a substructure for a floating oil or gas production platform, using the arrangement as described above.